1. Field of Invention
Embodiments of the invention relate generally to operation of uninterruptible power supply systems.
2. Discussion of Related Art
An uninterruptible power supply (UPS) is typically used to provide backup power to an electrical device, or load, while the primary power source, or mains, is unavailable. A conventional online UPS rectifies input power provided by an electric utility using a power factor correction circuit (PFC) to provide a DC voltage to a DC bus. The rectified DC voltage is typically used to charge a battery while mains power is available, as well as to provide power to the DC bus. In the absence of mains power, the battery provides power to the DC bus. From the DC bus, an inverter generates an AC output voltage to the load. Since the DC bus is always powered either by mains or the battery, the output power of the UPS is uninterrupted if the mains fails and the battery is sufficiently charged. One such UPS is called an online UPS, and typically includes a double conversion topology.
To improve efficiency, some conventional UPS systems include bypass mechanisms that permit input power to flow past the power conversion circuitry (e.g., the double conversion topology) to the load during normal operating conditions (e.g., while AC mains is available and stable), switching to online mode only when necessary. Operating in bypass mode is more efficient than operating in online (i.e., double conversion) mode because the power conversion circuitry consumes power due to losses even when it is not needed for powering the load. However, one drawback of this approach is that, while operating in bypass mode, the load is susceptible to voltage variations and fast electrical transients in the mains because the power conversion circuitry performs some degree of line conditioning. Therefore, it is preferable to route power through the converter at all times despite the power losses associated with operating in online mode.